As a result of steadily rising energy costs construction practices in residential housing have changed considerably inasmuch as residential dwellings are now more heavily insulated and usually contain high "R" value sheathing. As is well known in the industry, the "R" value of a material is a term used in connection with the performance of insulation and is determined by dividing the thickness of insulation by its thermal conductivity. The application of high "R" value sheathing which usually has low permeance has caused concern as a result of the changing moisture patterns that may occur in the wall. Excessive moisture in wall cavities can produce detrimental/destructive effects. The moisture may decrease the effectiveness of the cavity insulation and if a wall cavity remains wet for extended periods may result in the decaying and rotting of wood components of the wall. Under winter conditions condensation tends to accumulate on the insulation/sheathing or siding. The outdoor temperature and indoor humidity are the critical values. Indoor moisture moves toward the drier outdoors and will condense if the sheathing or siding are below the dew point temperature. This condensation may result in the buckling or warping of the siding or the blistering or peeling of the paint on the siding. Walls having higher "R" values have colder outside surfaces with greater condensation potential. The lower perm values of these walls also results in less moisture movement and consequently less opportunity for moisture to disipate through vapor diffusion.
Where the source of moisture is external, rain water or surface condensation (dew) penetrates the siding under the influence of the wind and capillary action which can pull surface water on the siding exterior up between the horizontal laps of the siding. The moisture can then migrate from between the laps of the siding and the siding/building paper interface both towards the exterior (into the wood siding towards the paint/finish siding interface) when a sheathing of relatively low permeability is used and towards the interior (into the sheathing material) when a sheathing of relatively high permeability is used.
Absorption of water into sheathing material of wood fiber based board, cellulose fiber based board and perlite based board not only lowers the "R" value but causes dimensional instability in the board as well. Dimensional instability in the board in turn causes the sheathing to warp, buckle and pull away from the nails holding it in place and in many instances may result in the total failure of the product.
Condensation occurs typically at a marked change of permeance such as at the interface of two different materials where these materials are below the dew point temperature. It is not significant that the two materials may be both highly permeable to water vapor, only that they are each of differing permeability. An example of this is when a spun bound olefin film covers fiber glass insulation on the cold side of a wall. The film is "permeable" however the fiber glass insulation is more permeable and condensation occurs at the film/insulation interface. How much moisture accumulates is dependent on the potential for evaporation at that interface. Consequently, at cold temperatures considerable moisture may collect with resulting damage to components of the wall if the moisture is allowed to remain there for extended periods of time. A difficulty with sheathing such as asphalt coated wood fiber based board, cellulose fiber based board, and asphalt coated perlite board, is their relatively low permeance. While these materials have a relatively large capacitance to accumulate moisture their relatively low permeability prevents them from giving up the moisture as the wall passes through its climatic cycle whether it be daily or seasonal and as a result the wall never has the opportunity to completely dry.
Thus, there has existed a definite need for an insulating board which in addition to having sufficient capacity to accumulate moisture has sufficiently high permeance to permit the moisture to escape the board under proper conditions.
It is an object of this invention to provide a new and improved insulation board that will meet these needs of capacitance and high permeance and yet provide improved R values.
It is another object of this invention to provide an insulating board having greater dimensional stability than comparable insulating boards.
Still another object of the present invention is to provide an insulating board having superior flame and smoke life safety characteristics.
Yet another object of the present invention is to provide an insulating board that possesses lower thermal conductivity (or higher "R"/inch) than previously attainable in insulation boards of comparable density.
Additional objects and advantages of the invention will be set forth in part in the description, or may be evident by the practice of the invention.